The Fallers are an alien race that are able to make almost-perfect copies of other species, after first absorbing them into their eggs. They are dropping eggs on the human-inhabited planet of Bienvenido, believing that the humans are vile animals who are denying the Fallers of their chance to reach The Heart. The eggs drop at a rate that strike fear into the hearts of the humans, such that a substantial portion of their society and economy is devoted to identifying and neutralising a Fall, as they call a crop of eggs falling to ground. But the rate of falling is not so great that the Fallers are seriously threatening to eliminate the humans, allowing human society to flourish.
Fallers drop their eggs which lure, trap and absorb humans. The absorbed human is copied by the egg, the result is a being that looks like the human but is inhabited by the consciousness of a Faller. Fallers also eat humans, not bothering to kill them first. The process by which Fallers absorb humans is almost as brutal.
A key passage that explains the nature of the Fallers occurs in the first chapter that takes the viewpoint of Slvasta in The Abyss Beyond Dreams. When asked by Slvasta "Monster, why are you so evil?", a Faller who had Slvasta in her power replied "You make us; we are formed by you, your body, your mind. This - what I am, the way I think - it is inherited from your kind. It is vile. You, your species, is animal, brutal, despicable. Once we have exterminated you, it will take a generation to breed you out of us. But we will be free in the end." Slvasta replies "You will never defeat us. Freak monster. The Heart is for us, not you. You will never be fulfilled." The Faller replies "We have been before. We will be again".
Near the end of The Abyss Beyond Dreams, Nigel Sheldon manages to secure an egg and probe its nature, finding that the eggs are artificial constructs, rather than a stage in the life-cycle of a natural species. Nigel learns that the Fallers were a space-faring species native to the Milky Way, that evolved into being that could shape their forms at will, even becoming able to fly between the stars unaided at a considerable fraction of light speed. The Fallers colonised new planets by raining their eggs down, copying the local species and supplanting them. But then one of their fleets was drawn into the Void. In their attempts to adapt their forms to accommodate the Void's basic drive to encourage species to fulfilment and absorb them into The Heart, some Fallers became Skylords.